docs: def get_singlerate(self, base, code):
          """Get a single rate, used as fallback"""
          try:
              url = self.onerate_url % (base, code)
              resp = get(url)
              resp.raise_for_status()
          except exceptions.HTTPError as e:
              self.log(logging.ERROR, "%s: problem with %s:\n%s", self.name, url, e)
              return None
          rate = resp.text.rstrip()
  
          if rate == 'N/A':
              raise RuntimeError("%s: %s not found" % (self.name, code))
          else:
              return Decimal(rate)